Not Everyone Calculates Sow Productivity Indexes the Same

It is important to ensure that the value of traits are calculated in the same way and are correct if we are to get accurate results from any evaluation.

Oct 19, 2011

A variety of sow software management programs are used to estimate the genetic parameters and breeding values across herds, so a close examination of the traits used and the emphasis on each trait is warranted. It is important to ensure that the value of traits are calculated in the same way and are correct if we are to get accurate results from any evaluation.

For example, all sow management software systems include the trait pigs weaned/sow/year (PW/S/Y). It is a measure of output (pigs weaned) expressed on a unit-of-cost basis, where the unit of cost is one sow in the herd for one year. However, different software systems may calculates PW/S/Y in different ways. From a mathematical standpoint, if you have a 1,000-sow herd and the PW/S/Y is 30, then you should have weaned 30,000 pigs over a one-year period. However, if you look at shorter time frames, there is great variability in PW/S/Y.

In order to smooth out the variation in PW/S/Y, the old traditional software standard, PigChamp-dos, calculates PW/S/Y as litters/sow/year (times) number of pigs weaned/litter. The litters/sow/year (L/S/Y) is calculated using a formula recommended by the Production and Financial Standards committee of the National Pork Producers Council. L/S/Y for a period of time = [sum of successful gestation days/sum of breeding female days] x [365/115]. When L/S/Y is calculated for a sow, if the sow is pregnant but has not farrowed, the gestation days are assumed to be “successful.” As we all know, not all sows that are bred actually farrow, so this is a somewhat optimistic estimate of L/S/Y. When this figure is used to calculate PW/S/Y, the number is a bit higher. The table below shows a comparison of the two ways to calculate PW/S/Y.

So which is the correct way to calculate PW/S/Y? The reason for the “optimistic” LSY is valid, but it is an overestimate of what was actually produced over the year. Since you can only sell actual pigs from litters that actually farrowed, most software programs have moved to the “actual” calculation.

Some commercial firms, such as genetic suppliers, use PW/S/Y numbers for marketing purposes. For the producer trying to make data-based decisions, it makes sense to use information based on actual data (i.e., litters farrowed and pigs weaned).